Input: gpio_keys - replace timer and workqueue with delayed workqueue

We do not need to roll our own implementation of delayed work now that we
have proper implementation of mod_delayed_work.

For interrupt-only driven buttons we retain the timer, but we rename
it to release_timer to better reflect its purpose.

Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>